You gain weight when the fat cells in your body either expand (because they are storing extra calories), or when they increase in number. Each of us is born with a certain number of fat cells, as determined by our genes. This impacts your weight for the rest of your life. However, if you overeat and do not get enough physical activity -- particularly during childhood and adolescence -- you can actually develop MORE fat cells.

When you lose weight, your fat cells contract -- but unfortunately you don't lose any! This is one reason why it is easy to gain weight back. This is also why it is important to keep children from becoming overweight in the first place.

When you don't burn as many calories as you consume, the body stores the extra calories in fat cells. These cells function as energy reservoirs, and they enlarge or contract depending on how you use this energy.

The most common cause of weight gain is fairly simple: your energy input (the calories you eat) is greater than your energy output (the calories you burn). In fact, even with a medical condition that may increase the likelihood of becoming overweight (see below), one of these two factors is generally present before you gain weight -- namely, you eat too many high-calorie, high-fat foods or you don't get enough physical activity.

There are, however, certain medical conditions that increase your chances of becoming overweight or obese:

See your doctor to determine if there are any underlying reasons for your weight gain. Your doctor can also make sure you are healthy enough to begin a weight loss program and check what activities you can safely do. Your doctor may tell you to avoid certain exercises that put too much stress on your body, like jogging or high-impact aerobics, particularly early on.

You also may have specific nutritional needs (for example, if you have diabetes). Your doctor will want to make sure you get adequate nourishment -- including a balance of important vitamins and minerals -- while you lose weight and keep it off.

If any of the following is true, it is particularly important that you involve your doctor in your weight loss decisions: